Patent GB-891373-A (GB)
Title: Improvements in or relating to electrical power supply equipment
Inventor: BENNETT GORDON CHARLES
IPC: H04M3/40
Filed: 1960-03-14
Abstract: 891,373. Converting; automatic current control systems. STANDARD TELEPHONES & CABLES Ltd. March 14, 1960, No. 8984/60. Classes 38 (2) and 38 (4). [Also in Group XL (c)] The output current of a ringing choke D.C. to D.C. converter is maintained constant by controlling the on-off ratio of the transistor switch in the D.C. input circuit. A D.C. input is connected to the primary of a transformer T1 through a paralleled pair of switching transistors XR1, XR2, to provide, by ringing choke conversion and rectification at MR1, MR2, two D.C. supplies of a higher potential. These supply a normally balanced load (the repeaters of a telecommunications cable) connected at the " output " terminals. A string of Zener diodes is included in each output line to provide stabilized output voltages for various auxiliary circuits, and the output is shunted by Zener diodes VL to limit the open-circuit voltage. Each output line also includes a resistor RV1 or RV2 to provide a signal proportional to output current. The switching transistors XR1, XR2 are controlled via a phase-inverting stage PI from a mono-stable transistor multivibrator MM, Fig. 3 (not shown). This is switched at regulator intervals to render transistors XR1, XR2 conductive by means of timing pulses generated at TPG. The length of time for which the multivibrator MM remains switched depends upon the potential to which a capacitor is charged through resistor R8, hence by varying the potential, the " on " time of transistors XR1, XR2 can be controlled. One end of resistor R8 is connected to a stable voltage obtained from point b on a Zener diode string energized by an auxiliary D.C. source derived from rectifier MR3. The other end is connected by a diode network MR5, MR6, MR7 to three alternative control potentials. The lowest potential, relative to point b, takes control and the other two diodes are blocked. The first control potential, tapped off a potential divider P1 from the auxiliary D.C. source, controls the output potential under no load conditions. The second, and normally operative, control potential is derived from the output of a first, differential amplifier DA1, which consists Fig. 2 (not shown), of an emitter-coupled pair of transistors in turn controlling a further transistor, the collector load of which is the resistor R6. The bases of the emitter-coupled pair are connected to opposite ends of resistor RV2, and the emitters to the negative end d of resistor RV1. Then, if the load is balanced, and the voltages across both BV1 and RV2 increase, or if the load is unbalanced so that the voltage across RV2 alone increases, the output of the amplifier decreases so that it assumes control and reduces the input to the converter. The third possible control potential is derived from the output of a second differential amplifier DA2, again consisting of an emitter-coupled, pair of transistors, this time responsive only to the voltage across RV1 so as to assume control if the load is so unbalanced as to increase the voltage across RV1 only.